1. Field
The disclosed embodiments relate to the field of aircraft propelled by turbojet engines.
The disclosed embodiments more particularly relate to an aircraft architecture comprising one of more jet engines located in the rear part of the aircraft.
2. Brief Description
In transport aircraft propelled by jet engines the arrangement of the engines is variable in particular depending on the number of engines and in practice also on the size of these engines, size introducing important constraints.
When the number of jet engines is even, generally two or four jet engines, exceptionally six or eight, the jet engines are most often fixed under the wing of the aircraft, according to a symmetrical arrangement with respect to the axis of the aircraft. This arrangement is very common at present, on civil transport jet aircraft, because of its good performance in terms of the aerodynamic integration of the aircraft and despite the constraints it imposes on the undercarriages in order to maintain a sufficient ground clearance of the jet engines.
Another arrangement of the jet engines consists in fixing the jet engines on the rear part of the fuselage, symmetrically on each side. This arrangement gives rise to difficulties with regard to certification when two jet engines must be fixed on each side of the fuselage of an aircraft having four jet engines and generates difficulties in the management of the position of the center of gravity of the aircraft, in particular with large-sized jet engines like the modern jet engines with high bypass ratios. For these reasons, only aircraft of modest size having two jet engines using relatively small jet engines are at present designed with this system of mounting jet engines on the sides of the fuselage at the rear of the aircraft.
When the number of jet engines is odd, the extra engine with respect to the even number is generally placed in the axis of the aircraft, in the rear zone of the fuselage, and the even number of jet engines are arranged symmetrically as described before.
The examples of transport aircraft having three jet engines are rather limited and there can be mentioned in particular the DC10, comprising two jet engines fixed under the wing and a third jet engine fixed on the vertical stabilizer at the rear of the fuselage, the L1011 also comprising two jet engines fixed under the wing and a third jet engine fixed in the rear part of the fuselage, the B727 which comprises two jet engines fixed on the sides of the fuselage at the rear and a third jet engine fixed in the rear part in the axis of the fuselage.
These configurations with three jet engines, and in particular the arrangement of the third jet engine in the axis of the aircraft, present problems which explain why they are seldom used despite the advantage that the use of three engines can provide rather than two or four engines.
A first problem is related to the aerodynamic integration of such jet engines with the aircraft. In fact, the proximity with respect to the fuselage of these air intakes induces losses of aerodynamic loads and distortions of the flow speeds field that are prejudicial to good efficiency of the jet engine thus placed.
A second problem, also very critical for companies operating the aircraft, relates to the maintenance of these jet engines. Placed higher and less accessible than under-wing jet engines, they necessitate special means for the daily or periodic checking operations and their installation and removal operations are much more problematic than for jet engines suspended under a wing, which proves to be penalizing in operation. Moreover, their specific installation necessitates a special design of the nacelles, different for example from the jet engines fixed under the wings, and therefore different spare parts for the nacelles, different maintenance manuals and different training for maintenance personnel.
Whatever the number of jet engines and the arrangement of these jet engines on the aircraft may be, changes in regulations require increasingly lower noise levels for civil aircraft and the conventional configurations of jet engines fixed under the wing of the aircraft or at the rear of the fuselage present difficulties because of the noise which is radiated by the jet engines either forwards, fan noise at the air intake, or rearwards, jet noise. Various means are used at present at the level of the jet engine in order to reduce the radiated noise: acoustic treatment of the surfaces of the jet engine, elongation of the air intake, mixtures of primary and secondary flows of the jet, etc.
Although effective, these means however have limits because of the consequences they have on the efficiency of the jet engine and on its mass.
Another complementary method, known for reducing the noise radiated towards the ground and for limiting the disturbance of people living near airports, consists in using a part of the structures of the aircraft for creating a mask between the noise source of the jet engine and the ground. Solutions following this principle have been considered, such as for example fixing the jet engines above the wing or above the fuselage between two vertical stabilizers as proposed by the patent FR2873096. In this last example, the fuselage makes it possible to create a mask with respect to the noise radiated by the air intake and the fuselage: the horizontal stabilizer and the vertical stabilizers partly mask the jet noise in the downward and lateral directions.
However, in these various arrangements, the efficiency of the rear jet engine is affected and the maintenance problems remain high.